Precision treatment of solid and liquid materials may be accomplished by heating the materials to a desired temperature at a location where treatment is desired. Examples of such heat treatment include soldering metallic parts, curing epoxy resins, removing plastic coatings from metals, and boring holes in solid materials.
Several techniques are currently used to perform precision heat treatment. One such technique is optical, using focused light, or lasers. The focused laser beam produces a high intensity spot on the material at the location where treatment is desired. Imaging systems are often used with laser material processing techniques to monitor the treatment process.
A laser treatment apparatus that uses video imaging to monitor the process commonly has the imaging device and the laser beam share the same optical axis. This ensures accurate monitoring of the treatment process by obtaining an image at the precise location where treatment occurs. In such a system, additional illumination is generally required for the imaging. Because the optical path of the imaging device is coextensive with the optical axis of the laser beam, conventional systems focus the illumination light onto material at the treatment location along this same axis.
FIG. 3 schematically illustrates a typical optical layout of such a prior art laser material processing system. The illustrated apparatus includes a laser 10, a video camera 11, and an illumination light 12. Laser 10 of FIG. 3 is a traditional gas or solid state laser which produces a laser beam 13 having a small divergence (typically less than 1 degree). Beam 13 from laser 10 is collimated by beam collimator 14. A beam expander (not shown) is sometimes disposed between laser 10 and beam collimator 14.
Dichroic mirror 15 deflects collimated beam 16 along optical axis 25 toward lens 20. Lens 20 is typically a microscope objective lens, sometimes of the "long-working distance" type (working distance being the distance from the target to the closest optical element). Lens 20 focuses beam 16 onto target 21. Target 21 is the material upon which treatment is desired.
Video camera 11 is used in such a conventional system to monitor the material treatment process. Video camera 11 receives the image of the treatment process from an optical path that extends perpendicularly from the treatment location on target 21, through dichroic mirror 15, through beam splitter 22, to video camera 11. The optical path of video camera 11 is thus the same as optical axis 25 of laser beam 16. A lens 23 may be disposed in the optical path in front of video camera 11.
Illumination light 12 is used to improve the quality of the image monitored by video camera 11. Light 24 from illumination light 12 is collected by simple lens 26 into light beam 27. Beam splitter 22 deflects light beam 27 onto lens 20 and target 21 at the material treatment location along the same optical axis 25 as that of laser beam 16 and video camera 11.
In the design of any material treatment process using video imaging, several desirable parameters, such as small laser beam spot size at the target and long working distance, must be balanced. With the type of system illustrated in FIG. 3, lens 20 is shared by the optical path 25 of laser 10, video camera 11, and illumination light 12. Lens 20 must therefore fulfill several constraints: it must focus the laser beam 16 to the smallest feasible spot size onto target 21, it must provide adequate working distance, and it must provide well-corrected, flat-field imaging over the desired field-of-view for video camera 11. (In systems such as that depicted in FIG. 3, the spot diameter is typically in the range of 0.001-0.1 mm, and the working distance is in the range of 1-15 mm.)
Because the traditional gas or solid state lasers used as laser 10 in the illustrated prior art system produce laser beams having a small divergence, it is possible for lens 20 to be designed and positioned such that an acceptable balance of the desired parameters in the system are produced.
When laser diodes are used instead of these traditional lasers, however, the higher divergence of the laser beam generated by the laser diode makes the design trade-offs in the material treatment system more difficult to resolve. Commercially available fiber-coupled laser diode arrays provide 15 W out of a fiber bundle of 1.5 mm diameter with a half-angle divergence of about 7 degrees. In applications such as soldering, it is desirable to use a beam spot of about 1 mm diameter, with a working distance of at least 20 mm. Commercial objective lenses for use as lens 20 are not suitable to provide these parameters, and it is not economical to "scale-up" an objective design. If lens 20 is replaced by a lens system which is less well-corrected than an objective, the lens will not give adequate flat-field imaging for the camera. The prior art system of FIG. 3 is thus unacceptable when laser 10 is a laser diode source.
In addition, the prior art material treatment system of FIG. 3 has the drawback that illumination light 27 reflects from perpendicular surfaces, such as lens 20 or flat metallic regions of target 21. This may result in undesirable bright spots in the image captured by video camera 11.